1. Field of the Invention
This invention relates to zoom lenses suited to be used in photographic cameras for 35 mm film, video cameras or still video cameras and, more particularly, to wide-angle high range zoom lenses. Still more particularly, it relates to improvements of the compact form of the entire lens system along with high optical performance by setting forth appropriate rules of design for the construction and arrangement of the constituent lenses in each of the lens groups and by applying aspheric surfaces of appropriate shapes to prescribed lens surfaces.
2. Description of the Related Art
It has been known to provide a zoom lens in which the leading lens group is negative in the refractive power as one of the zoom types. This so-called "negative lead" type is relatively feasible for widening the angle of field and has found its use in many wide-angle zoom lenses of not less than 70.degree. in the maximum angle of field.
For example, Japanese Patent Publication No. Sho 59-16248 proposes a zoom lens having a first group of negative refractive power and a second group of positive refractive power, totaling two lens groups, wherein zooming is performed by varying the separation between these two groups. This is usually called the "short" zoom lens.
Other Japanese Laid-Open Patent Applications Nos. Hei 2-72316 and Hei 3-233422 propose 3-group wide-angle zoom lenses of which the first group, when counted from the object side, is of negative refractive power, the second group is of positive refractive power and the third group is of negative refractive power, wherein these groups are all made movable for zooming.
Another Japanese Laid-Open Patent Application No. Hei 2-72316 proposes a 4-group zoom lens of which the first group, when counted from the object side, is of negative refractive power, the second group is of positive refractive power, the third group is of positive refractive power and the fourth group is of negative refractive power, wherein these four groups are all made movable for zooming.
Meanwhile, as another zoom type, there has been known a zoom lens comprising, from front to rear, a first group of positive refractive power and a second group of negative refractive power, totaling two lens groups, both of which are made movable for zooming. Such a so-called 2-group zoom type is feasible for minimizing the bulk and size of the entire lens system, so that it has been used in many leaf shutter cameras or like compact cameras. Since, in the 2-group zoom type, all the function of varying the focal length is laid on only one lens group (that is, the second group), many zoom lenses of this type have their ranges limited to 1.6 to 2 or thereabout. If the 2-group type is applied to form a zoom lens of too much increased range, the lens system suffers from a rapid increase of the size. Moreover, it becomes difficult to keep the optical performance at a high level.
With the 2-group zoom lens, the first group may be divided into two parts of positive refractive powers. The total number of lens groups amounts to 3 and the plus-plus-minus refractive power arrangement is thus formed to aim at a great increase of the range. Such a 3-group zoom type is proposed in, for example, Japanese Laid-Open Patent Applications Nos. Hei 3-73907, Hei 3-282409, Hei 4-37810 and Hei 4-76511.
When the 3-group type is applied to achieve a zoom lens of wider maximum semi-angle of field than 35.degree., the entrance pupil is caused to vary its position to a larger extent with zooming. For the range to increase, therefore, it becomes difficult to suppress the variation of aberrations with zooming to a minimum.
In general, zoom lenses of the negative lead type are relatively easily amenable to widen the angle of field. However, to fulfill the requirements of increasing the maximum angle of field to more than 70.degree. and of obtaining good optical image quality over the entire area of the image frame at once, there is a need to find out appropriate rules of design for the refractive power arrangement of all the lens groups and the forms of the constituent members in the lens groups. If the refractive power arrangement of the lens groups and their forms are inappropriate, the variation of aberrations with zooming increases so greatly that good stability of high optical performance is hardly maintained throughout the entire zooming range, no matter how many constituent lenses may be used.
For a zoom lens to have not only wider angles of field, but also an increased range, the use of aspheric surfaces is very effective to achieve minimization of the size of the entire lens system and improvement of the optical performance. In this regard, however, to make a determination of what lens surfaces to select for introduction of an aspheric surface is very important, since it is depending on this that the aspheric surfaces produce greatly different effects of correcting aberrations. If the aspheric sphere is not introduced into the appropriate lens surface, effective correction of aberrations becomes difficult to do.
For example, the before-mentioned Japanese Laid-Open Patent Applications Nos. Hei 3-282409, Hei 4-37810 and Hei 4-76511 show that the positive lens in the second lens unit is selected to introduce an aspheric surface of such shape that the positive refractive power gets progressively weaker as the distance from the optical axis increases. In any of these zoom lenses, the selected one of the lens surface for introduction of the aspheric surface takes its place at a considerable axial distance from the stop so that the height of incidence of either the on-axial ray or the off-axial ray on that selected lens surface differs with different focal lengths. This phenomenon is used to correct the variation of aberrations. However, as the maximum angle of field widens and the range also increases, the ranges of variation of spherical aberration, for example, and of off-axial aberrations are caused to increase greatly. So, it becomes difficult for this aspheric surface to correct the se aberrations simultaneously.
U.S. Patent No. 5,069,536 discloses a 3-group zoom lens of plus-plus-minus power arrangement, wherein an aspheric surface is introduced into that lens surface in the second lens unit of positive refractive power which lies near the stop, is concave toward the object side and has a negative refracting function. However, if, in this zoom lens, a widening of the maximum angle of field and a great increase of the range are to be achieved, the total zooming movement of each of the lens groups increases rapidly and also the front lens members in the first lens group get a large diameter. Thus, a long bulky lens system results. Moreover, as the refractive power of each of the lens groups is strengthened, the range of variation of aberrations increases, which is difficult to correct well by that aspheric surface.
Japanese Laid-Open Patent Application No. Hei 3-49614 shows a wide-angle high range zoom lens comprising four lens groups of plus-plus-plus-minus refractive power arrangement, or four lens groups of plus-minus-plus-minus refractive power arrangement. However, the total number of constituent lenses is too large and the aspheric surface does not effect sufficient results.
Japanese Laid-Open Patent Application No. Hei 3-73907 shows a zoom lens comprising three lens groups of plus-plus-minus refractive power arrangement, wherein at least that positive lens in the second lens group of positive refractive power which lies near-the stop is selected to introduce an aspheric surface of such shape that the positive refractive power gets progressively weaker as the height from the optical axis increases. With the help of this, a wide-angle zoom lens is achieved. However, the second group has a large number of lens elements and the aspheric surface in the same lens group does not sufficiently contribute to improvements of the compact form.
Japanese Laid-Open Patent Application No. Hei 3-233422 shows a zoom lens comprising three lens groups of minus-plus-minus refractive power arrangement, wherein a plurality of aspheric surfaces are used in the third lens group. However, the optical total length is of the order of 66 mm even at the wide-angle end. So, it can hardly be said that the total length of the whole lens system is short. Moreover, if a further widening of the maximum angle of field is done, good correction of aberrations is difficult to perform over the entire zooming range.